This section introduces aspects that may be helpful in facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In many wireless systems, a base station adapts the transmission mode for each user (e.g. a mobile station) according to its current channel state. Depending on the system's physical layer (PHY), such an adaptive downlink scheme may define the transmit rate, power, frequency band, and/or precoding matrix. To do so, the base station requires accurate Channel State Information (CSI) of the downlink towards each particular user. In many systems, e.g., LTE and IEEE 802.16, the Uplink and Downlink channels cannot be assumed to be fully reciprocal. Hence, CSI is obtained at the User Equipment and fed back to the base station via the wireless uplink.
One major problem of the CSI feedback from the user equipment is the feedback delay in conjunction with channel ageing. Inherently, mobile communication systems have a certain feedback delay. When the mobile terminal moves (or channel reflectors) the radio channel changes during that feedback delay (“channel ageing”) and thus the feedback information for the downlink transmission is no more accurate.
As indicated above, mobile communication systems have an inherent feedback delay. For instance, in 3GPP LTE/LTE-A, this delay cannot be below 4 ms, for the following reason: 1 ms is required for sending downlink pilots, 1 ms is required for channel estimation, feedback calculation and waiting for the next appropriate uplink sub-frame, 1 ms is required for uplink feedback transmission, and 1 ms is required for uplink reception, feedback extraction and waiting for the next downlink sub-frame in order to use the obtained feedback information. Additional processing times increase the feedback delay. It is shown, e.g. in [3], that this delay severely degrades the feedback performance.
One way of dealing with this problem aims at making the processing (for channel estimation, decoding etc.) as fast as possible. This is a straightforward approach which is handled by Moore's Law, but cannot overcome inherent bounds (like 4 ms for LTE).
A second solution would be to completely change the air interface frame structure—e.g. bringing the LTE sub-frame length of 1 ms down to 0.5 ms or even less. This approach helps to reduce the delays, but will at a certain point reach its limits: For instance, cyclic prefix length coping with multipath propagation cannot be arbitrarily reduced for a certain cell size, otherwise inter-symbol and inter-carrier interference will degrade the performance. A second limit is the coding gain: Reducing sub-frame lengths reduces codeblock sizes and thus reduces the coding performance.
A second major problem of CSI feedback from a user equipment is the resulting feedback overhead, which reduces the available throughput of the actual uplink data transmission. For mitigating this problem, it has been proposed to reduce the CSI overhead based on transformations and concentration on the strongest channel taps, as shown in [2].